Waste of energy
It's one thing to lose your investment in a lotto ticket. You are the only person that loses. That's fair.
It's another thing to generate pollution and waste energy every time you lose. We all lose energy and air cleanliness. This is not fair because we all have to pay for your greed.
A lot of miners compete every 10 minutes to see who can solve a problem by calculating a hash before anyone else in the world. The winners use a tiny amount of electricity to calculate the winning hash and are awarded some new bitcoins for their trouble.
The amount of power used to calculate the hash by the competing losers is enormous The people who are mining bitcoin lose enough money to power a country the size of Switzerland in the competition.
It does not need to be hard or use a lot of power to calculate the hash. It just is because there is an increasing difficulty level artificially added by the system by the creator. He felt that if it did not get harder people would not mine for bitcoins. So this increasing difficulty has been dealt with by miners buying faster and faster machines to compete with each other. Although the newest and best machines perform more work for each unit of energy they use, many more people are buying machines and throwing them into the competition meaning more power being used just to compete.